1. Technical Field
This invention relates generally to metal casting control devices and methods for controlling the pour of molten metal into molds and, more specifically, to an apparatus and method for controlling the positioning of a stopper rod of a bottom pour casting vessel.
2. Related Prior Art
Accurate control of the opening and closing of a stopper rod of a bottom pour casting vessel is essential for producing high quality metal castings. A typical stopper rod control mechanism in present day use and manufactured by the assignee of the present invention for pouring molten iron is shown in FIG. 1 and comprises a bottom pouring vessel having a bottom nozzle and a stopper rod extending into the vessel for communicating with the nozzle. A hydraulic cylinder remotely controls the movement of the stopper rod through a parallelogram linkage mechanism having six pivot points A programmable controller controls actuation of the hydraulic cylinder through a digital proportioning valve. The cylinder is provided with a digital encoder which measures movement of a rod of the cylinder commanded by the digital valve.
With all bottom pour casting systems, it is imperative that the movement and position of the stopper rod be precisely controlled since it is the stopper rod that governs the flow of molten metal through the nozzle. The prior art system of FIG. 1 relies on the digital encoder of the hydraulic cylinder to sense the position and stroke of the stopper rod. A problem arises, however, in that each pivot point of the linkage mechanism has a certain amount of "slop" or movement incorporated in its design which is necessary to avoid tight fits in the high temperature environment in which the mechanism operates. The accumulated slop of the pivot points can translate to an error of 0.30 to 0.70 inches between actual movement of the stopper rod and the movement sensed by the encoder. The solution heretofore has been to add a "slop factor" into the logic of a programmable controller to compensate for the error. This factor anticipates the amount of slop and compensates in terms of the stroke commands provided to the stop. There is a problem, however, in that the amount of slop varies with temperature. Varying metal level in the vessel adds to the control problems by changing the buoyancy forces on the stopper rod. Because of these problems, it is presently necessary to periodically update the slop factor thereby requiring constant monitoring of the pouring system.
The controller of the prior art pouring apparatus of FIG. 1 is programmed to raise and lower the stop rod in relation to the nozzle according to a predetermined pouring schedule characteristic of the mold being poured. Each casting cycle begins with the stopper rod in a fully closed position against the nozzle. The controller controls movement of the stopper rod according to the pouring schedule in relation to the fully closed position of the stopper rod. A problem rises, however, in that over time slag and other impurities build up on the nozzle which changes the actual position of the stopper rod when in the fully closed position. This adversely affects the ability of the prior art control system to control movement of the stopper rod since the changing fully closed reference position of the stopper rod is not taken into account.
The U.S. Pat. No. 4,953,761 to Fishman et al, granted Sep. 4, 1990 recognizes slag formation on the nozzle but proposes to resolve such problems by increasing the seating force of the rod so that the slag can be crushed off the nozzle enabling the stopper rod to regain its original fully closed reference position before build up. Applying excessive force to the nozzle, however, can damage both the nozzle and the rod and hence such a practice is to be avoided.